Heat-shrinkable multilayer film

Abstract

A polyamide resin-based heat-shrinkable multilayer film is caused to satisfy various properties required of a packaging material, especially a food packaging material, at high levels, and particularly exhibit a highest level of heat-shrinkability. The multilayer film comprises at least three layers including an outer surface layer (a) which comprises a polyester resin or a polyolefin resin; an intermediate layer (b) which comprises a mixture of 85-60 wt. % of an aliphatic polyamide resin, and 15-40 wt. % of an aromatic copolyamide resin which is a copolymer of aliphatic diamine / isophthalic acid and aliphatic diamine / terephthalic acid, and an inner surface layer (c) which essentially comprises a copolymer of ethylene and alpha-olefin having a density below 0.915. The heat-shrinkable multilayer film has been biaxially stretched and then heat-treated to have hot-water shrinkabilities at 80° C. of at least 30% in each of longitudinal / transverse directions and at least 35% in at least one of longitudinal / transverse directions.

Description

FIELD OF THE INVENTION AND RELATED ARTThe present invention relates to a heat-shrinkable multilayer film suitable for use as a heat-shrinkable packaging material and comprising at least three layers including an outer surface layer (a) formed of a thermoplastic resin, an intermediate layer (b) formed of a polyamide resin, and an inner surface layer (c) formed of a heat-sealable resin.Hitherto, it has been widely practiced as a packaging and processing technique for content materials, inclusive of foods such as raw meat, ham and sausage, principally, and other products, to form a casing, such as a bag or a pouch, from a heat-shrinkable multilayer film, and then fill the casing with such a content material, or to automatically package such a content material per se or a tray loaded with such a content material while simultaneously forming a casing from a heat-shrinkable multilayer film.As heat-shrinkable films for such use, many films comprising a polyamide resin as a principal resin ...

AI Technical Summary

Problems solved by technology

However, a single layer of polyamide resin cannot satisfy many other properties required of a packaging film.

(b) For Example, a heat-sealing method is adopted in many cases for hermetically sealing a packaging film after packaging the content material, whereas a polyamide resin does not generally have a good heat-sealability.

(d) A polyamide resin has a certain degree of gas-barrier property, but it is preferred to dispose a gas-barrier resin layer in addition to the polyamide resin layer for packaging of a content material of which the degradation is extremely undesirable.

(f) A film imparted with a heat-shrinkability through biaxial stretching is liable to cause some degree of shrinkage during a period of storage thereafter until its use for packaging or when heated to a temperature of at most 50.degree. C., thus causing wrinkles or size change.

This is liable not only to impair the appearance of the product film but also to cause a trouble in an automatic packaging step.

This heat treatment however also functions to reduce the heat-shrinkability of the film after biaxial stretching.

Accordingly, it is not easy to provide a laminate film structure capable of exhibiting stable inflation performances (capability of forming a stable inflation bubble and acquiring a high heat-shrinkability owing to a biaxial stretching effect of the inflation) for a wide range of bubbles (i.e., a wide range of lay-flat widths) receiving different biaxial stretching stresses.

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